This invention is concerned with determining radioactivity by liquid scintillation counting. In particular the invention is directed to effecting such counting by efficiency extrapolation methodology.
In known liquid scintillation counters, the activity of beta-emitting radioisotopes is measured by the use of quench curves, a plot of the counting efficiencies of known standards vs some quench measuring parameter. The activity of an unknown sample is obtained by measurement of its quench level, which gives the counting efficiency from the quench curve and permits calculation of activity from ##EQU1## This method requires the preparation and counting of several quenched standards of the same radioisotope as the unknown sample. The preparation and counting of such series is time-consuming and demands good laboratory technique.
The method of efficiency extrapolation requires preparation and counting of only one standard sample to enable the recovery of disintegrations per minute of almost all beta, and beta-gamma and alpha emitters. By use of a second standard, the manual method is extended to isotopes with energy below Carbon-14.
In a conventional use of the efficiency extrapolation methodology, at least two restrictions are present: a limitation to narrow quench ranges, for example, greater than about 80% counting efficiency for C-14 and, inapplicability to tritium.
Efficiency extrapolation has been adapted for use in liquid scintillation counting (M. Takiue and H. Ishikawa, Nucl. Instr. and Meth. 148, 157 (1978). Takiue and Ishikawa have further disclosed a computerized technique on a liquid scintillation counter (PCT/JP84/00213, filed Apr. 24, 1984, and Ishikawa, Takiue, Aburai, Int. J. Appl. Isot. Vol. 35 No. 6, pp. 463-466, 1984). In U.S. Pat. No. 4,060,728 (Horrocks) coupled the technique with direct pulse height measurements produced by external gamma irradiation at a minimum of two different quench levels.
There are at least two drawbacks of the computerized method:
1. It is inapplicable for radioisotopes with energies less than Carbon-14.
2. It depends upon 6 specific fixed regions of the spectrum of the standard, thus limiting its applicability to a small quench range.
The Horrocks method requires at least two measurements of the unknown sample at different quench levels. This requires time and the complexity of adding optical filters to the liquid scintillation counter or chemical quenching agents to the sample. In the latter case, the unknown sample composition is destroyed.
There is accordingly a need to provide for a non-limiting dynamic simple efficiency extrapolation methodology applicable to radiosotopes with energies below C-14.